What causes mittelschmertz (ovulation pain)?

I used to just get a cramp in one side, now I get pelvic and back pain too. I also suffer painful periods, mainly the first couple days. Doc just gave me pain pills(naproxen 550) and told me I was constipated. They work sometimes, but not much. Heating pad dont help. I already plan to switch docs, and want to ask for hysterectomy(already got tubes tied), but cant afford medical bills now, we just got a car. Help plz?